This is a Division of application Ser. No. 10/352,199 filed Jan. 28, 2003. The entire disclosure of the prior application is hereby incorporated by reference herein in its entirety.
1. Field of Invention
The present invention relates to a liquid crystal display and an electronic device. More specifically, the invention relates to a technique of obtaining a high-contrast display having a wide viewing angle in a transflective liquid crystal display that performs display in both a reflective mode and a transparent mode.
2. Description of Related Art
Since reflective liquid crystal displays have no light sources, such as a backlight, they consume low power, and thus can be used for various portable electronic devices. However, the reflective liquid crystal displays perform display using outside light, such as sunlight and illumination light. Thus, these displays are subject to low visibility in a dark place. Therefore, the related art includes liquid crystal displays capable of making display visible using outside light in a light place, as in general reflective liquid crystal displays, and using an inside light source, such as a backlight, in a dark place. In other words, such liquid crystal displays employ a reflective and transparent display system, thereby allowing clear display even in low light while reducing power consumption by switching the display system between the reflective mode and the transparent mode depending on the surrounding brightness. Hereinafter, in this specification, liquid crystal displays of this type are referred to as “transflective liquid crystal displays.”
Such related art transflective liquid crystal displays include a liquid crystal display having a structure in which a liquid crystal layer is sandwiched between an upper substrate and a lower substrate. A reflective film having a light-transmitting window in a metallic film made of aluminum or the like is provided on the inner surface of the lower substrate, and this reflective film functions as a transflective film. In this case, in a reflective mode, outside light that has entered from the upper substrate passes through the liquid crystal layer, is then reflected by the reflective film, again passes through the liquid crystal layer, and outgoes from the upper substrate, thus contributing to display. On the other hand, in a transparent mode, light from the backlight, which has entered from the lower substrate, passes through the liquid crystal layer from the window of the reflective film, and then emerges from the upper substrate to the exterior, thereby contributing to display. Accordingly, in the reflective-film formed area, the area which has the window serves as a transparent display area and the other area serves as a reflective display area.
Liquid crystal alignment modes include a twisted nematic (hereinafter “TN”) mode in which liquid crystal molecules exhibit a twisted alignment substantially parallel to the substrate surface and vertical to the substrate; and a vertical alignment mode in which liquid crystal molecules exhibit vertical alignment, under a no voltage applied state. Although, in the related art, the TN mode can be viewed as reliable, the related art also includes liquid crystal displays that in the vertical alignment mode can provide some excellent characteristics.
For example, in the vertical alignment mode, since the state in which the liquid crystal molecules are aligned vertically to the substrate surface (there is no optical retardation as viewed from the normal) is used as black display, the black display is superior in quality, thus providing high contrast. In vertical-alignment LCDs which are superior in front contrast, the range of viewing angle in which a fixed contrast can be obtained is wider than that of the horizontal-alignment-mode TN liquid crystal. Furthermore, employing an alignment dividing (multidomain) technique of dividing the alignment orientation of a liquid crystal in pixels provides a remarkably wide viewing angle.
In the transflective liquid crystal display with the aforesaid structure, the retardation of the liquid crystal in the reflective display area is expressed by: 2×Δn·d, because the incident light passes through the liquid crystal layer two times and then reaches the observer, where the thickness of the liquid crystal layer is d, the refractive index anisotropy of the liquid crystal is: Δn, and the retardation of the liquid crystal which is expressed as their integrated value is: Δn·d. On the other hand, the retardation of the liquid crystal in the transparent display area is expressed by: 1×Δn·d, because the light from the backlight passes through the liquid crystal layer only once.
As described above, when the alignment of the liquid crystal molecules of the liquid crystal layer is controlled, even with the structure having different retardation values in the reflective display area and in the transparent display area, an electric field has been applied to the liquid crystal at the same driving voltage in both display modes. In such a case, when the liquid crystal with different display modes, in other words, the liquid crystal with different retardations between the transparent display area and the reflective display area is aligned at the same driving voltage, it poses a problem of obtaining no high-contrast display. In order to address or solve the problem, a liquid crystal display is disclosed in Japanese Unexamined Patent Application Publication No. 11-242226 that has a structure having different thicknesses of the liquid crystal layer in the transparent display area and in the reflective display area.